In general, a fuel cell system generates electrical energy by supplying air and hydrogen to a fuel cell and using an electrochemical reaction of the hydrogen and oxygen generated by the fuel cell. For example, a fuel cell system is used to operate driving sources such as an electric motor in vehicles, ships, trains, and airplanes.
The fuel cell system includes a fuel cell stack, a hydrogen supplier that supplies hydrogen to the anodes of the fuel cells, an air supplier that supplies air to the cathodes of the fuel cells, and a heat/water control unit that removes heat and water, which are reaction by-products from the fuel cells, and controls the operational temperature of the stack.
In a polymer electrolyte membrane fuel cell, an appropriate amount of water is required for smooth operation of an ion exchange membrane of a membrane-electrode assembly (MEA), so fuel cell systems have a humidifier that humidifies a reaction gas that is supplied to stacks.
Such a humidifier humidifies air supplied from an air supplier, using water in high-temperature and high-humidity air discharged from the cathodes of fuel cells, and supplies the humidified air to the cathodes of the fuel cells.
Further, the fuel cell system includes a hydrogen recycler that mixes the hydrogen discharged from the anodes of fuel cells with the hydrogen supplied from a hydrogen supplier, and supplies the mixture back to the anodes.
While a fuel cell system is operated, impurities such as nitrogen and vapor are accumulated on the anodes of fuel cells, so the concentration of hydrogen decreases, and when the concentration of the hydrogen excessively decreases, cell separation may occur in the fuel cell stack.
In order to solve these problems, when the fuel cell system starts and operates, a purge valve periodically opens and impurities are discharged with hydrogen from the anodes, thereby maintaining the concentration of hydrogen at the anodes at a predetermined level or more.
When the anodes are purged by opening the purge valve, the anodes discharge hydrogen with impurities and a purge gas flows into a humidifier together with the air discharged from the cathodes.
Then, the vapor in the impurities is used as a source for humidifying a reaction gas needed for an electrochemical reaction of the fuel cells and the gas including hydrogen and nitrogen is discharged with air to the atmosphere through an exhaust system.
According to the above hydrogen purge type, the hydrogen discharged from anodes is mixed with the air discharged from the cathodes and then discharged to the atmosphere through an exhaust system, thereby achieving an effect of reducing the concentration of purge hydrogen by diluting the hydrogen with air.
Further, when a fuel cell system starts or stops or when a fuel cell vehicle equipped with a fuel cell system is in an idling condition (for example, a fuel cell vehicle is in an ISG condition), a large amount of hydrogen crossing over from the anodes to the cathodes of the fuel cells through a membrane is discharged.
The hydrogen is discharged with air to a humidifier from the cathodes of the fuel cells and is diluted with air through the humidifier, so the hydrogen is discharged to the atmosphere through an exhaust system with the concentration reduced.
However, as described above, although in the related art the concentration of hydrogen to be discharged is slightly reduced by mixing the hydrogen with air discharged from cathodes in a humidifier in accordance with the operation conditions of fuel cell systems, it is difficult to sufficiently mix the hydrogen and air, so the concentration of the hydrogen is not remarkably decreased.
Accordingly, in the related art, since the concentration of hydrogen discharged from fuel cell systems is not effectively reduced and there is quite a possibility of discharging hydrogen that has not been diluted, the discharged hydrogen having concentration over a predetermined level may cause ignition and explosion depending on the operation conditions of the fuel cell systems.
In order to prevent this problem, a method to discharge hydrogen after reducing the concentration of the hydrogen to be discharged through an exhaust system to a predetermined level or less is necessarily applied to the fuel cell system.
Recently, in order to prevent the possibility of ignition and explosion by hydrogen discharged from fuel cells, the concentration of hydrogen that fuel cell systems discharged to the atmosphere through exhaust systems is regulated less than 8% at the maximum and less than 4% per 3 second on the average by domestic regulations and relevant laws in the global technical regulation (GTR) about vehicle.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention, and therefore, it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.